Sensors

A sensor is an object whose purpose is to detect events or changes in its environment and then provide a corresponding output. A sensor is a type of transducer and may provide a variety of output but typically use electrical or optical signals.

Different types of sensors:

• Pressure sensor

• Ultrasound sensor

• Humidity sensor

• Gas sensor

• PIR Motion sensor

• Acceleration sensor

• Displacement sensor

A sensor basically consist of:

• Limit switches

• Photoelectric

• Inductive

• Capacitive

• Ultrasound sensors

All types of sensors can be classed as two kinds:

• Passive sensors

• Active sensors

All sensors have three terminals:

• Vcc- to power up the sensor

• GND – to provide a fixed negative reference

• Output – analog, output of the sensor

Active sensors require an external power supply to operate and is called an excitation signal which is used by the sensor to produce the outputsignal.

Unlike an active sensor, a passive sensor does not need any additional energy source and directly generates an electrical signal in response to an external stimulus.

Passive sensors are known as direct sensors which change their physical properties such as resistance, capacitance or inductance.

Terminology:

 Range – every sensor is designed to work over a specified range

 Zero - is a value ascribed to some defined point in the measured range

 Zero Drift –results from changes of temperature, electronic stabilizing or aging of  the transducer or electronic components

 Sensitivity – of a sensor is defined as the change in output of the sensor per unit  Change in the parameter being measured. The factor may be constant over the  range of the sensor (linear) or it may vary (nonlinear)

 Resolution – is defined as the smaller change that can be detected by a sensor.

 Response – the time taken by a sensor to approach its true output when subjected  to a step input.

 Linearity – output that is directly proportional to input over its entire range.

 Hysteresis-is the characteristics that a transducer has in being unable to repeat  faithfully.

 Calibration – if a meaningful measurement is to be made, it is necessary to measure the output of a sensor in response to an accurately known input- this process is known as calibration. The devices that produce the input are described as calibration standard.

 Span – a dynamic range of stimuli which may be converted by a sensor is called a Span or an input full scale (FS). It represents the highest possible input value which can be applied to the sensor without causing unacceptable large inaccuracy.

 Accuracy-is a very important characteristic of a sensor. Accuracy really means Inaccuracy. Inaccuracy is measured as a ratio of the highest deviation of a value represented by the sensor to the ideal value.

There are certain features that must be considered when choosing a sensor:

• Accuracy

• Environmental condition – limits for temperature humidity

• Range – measurement limit of sensor

• Calibration –this is essential for most of the measuring devices as the reading changes with time

• Resolution –the smallest increment detected by the sensor

• Cost

• Repeatability – the reading that varies is repeatedly measured under the same environment.

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